1. Field of the Invention
The present invention relates to a catalyst comprising a clay belonging to the family of smectites, containing multi-metal pillars; as well as to a process using such a catalyst for the alkylation of aromatic hydrocarbons by means of long chain linear olefins.
2. Description of the Background
The alkyl aromatic compounds are useful in a plurality of commercial applications. Among these, the most important application is their use in the preparation of biodegradable synthetic detergents. Processes for preparing aromatic compounds using catalysts of Friedel-Crafts type, such as, e.g., AlCl.sub.3, BF.sub.3, H.sub.2 SO.sub.4 and HF, has been long known. Unfortunately, these catalysts display, obviously, considerable drawbacks, because they cause corrosion problems in the used materials, as well as environmental problems associated with the disposal of the processing wastes.
Subsequently, some solid catalysts were found which are useful for performing the alkylation of aromatic hydrocarbons with olefins, yielding a production of comparable quality to the preceding one, however without the above said environmental and corrosion problems. Therefore, suitably treated zeolites and clays were used for that purpose. The latter can be both from natural sources and synthetic, exchanged with metal cations. For example, in U.S. Pat. No. 4,460,826, a natural or synthetic tri-octaedral clay exchanged with metal cations is used for alkylating benzene with long chain olefins. U.S. Pat. No. 4,499,319 claims the use of layered clays with laminar structure, such as montmorillonite, exchanged with such metal cations as chromium and aluminum, for the alkylation of aromatic compounds with alkylating agents containing less than 6 carbon atoms. Cation-exchanged and then suitably activated synthetic clays useful for alkylating aromatic compounds, are disclosed in U.S. Pat. No. 4,075,126. In EP-353,813, for the catalytic alkylation of aromatic hydrocarbons with olefins, either natural or synthetic zeolites, amorphous silico-aluminas, clays or their mixtures, possibly submitted to ionic exchange with aluminum, chromium or rare earths salts, are used.
However, metal cation exchanged clays showed poor heat stability. Therefore, as alkylation catalysts, modified clays were subsequently used which are referred to as "pillared clays", which, as compared to the above disclosed materials, are also stable at high temperatures.
These materials are prepared by starting from either synthetic or natural clays, such as, e.g., smectites, vermiculites or bentonites. The clays are constituted by layers of semi-crystalline alumino-silicate bonded to each other by electrostatic Van der Waals forces. The anionic charges on silica layers are neutralized by cations situated inside the interlaminar spaces. These cations, usually Na.sup.+ and Ca.sup.2+, can be exchanged with monomeric, oligomeric or polymeric species deriving from metal hydroxides, such as, e.g., hydroxo-polymeric cations EQU [Al.sub.13 O.sub.4 (OH).sub.24 (H.sub.2 O).sub.12 ].sup.7+ or [Zr(OH).sub.2 H.sub.2 O].sub.4.sup.8+.
Such cations act as a separation system for crystalline silico-aluminate planes, i.e., as a pillaring system. EP-83,970 discloses the use of a clay in which alumina pillars are anchored inside the interior of the laminar structure, for benzene alkylation with light olefins. In U.S. Pat. No. 5,034,564, a pillared clay containing an oxide of a metal selected from Al, Zr, La, Ce and Ti as spacer element for crystalline planes, co-extruded with a binder, is used in aromatic hydrocarbons alkylation reactions.
In Applied Catal., 14, 69-82 (1985) by M. L. Occelli, an Na montmorillonite layered with a system of aluminum oxide clusters, is compared to other catalyst types in toluene ethylation reaction.
These catalysts should display a high heat stability, also in the presence of water, in order to be able to withstand the regeneration treatments which are carried out in the presence of an oxidizer agent, such as air or oxygen, and which cause water to be simultaneously formed.
Catalysts with improved heat stability both when dry and in the presence of moisture are disclosed in U.S. Pat. No. 4,248,739, relating to the use of high molecular weight complex cations for clay pillaring, and in U.S. Pat. No. 4,963,518, in which the stabilization treatment of pillared clays by means of a silylating agent is disclosed. A method to increase the distance between clay layers, with the stability of clay being retained, consists in pretreating clays with non-ionic organic compounds, such as polyvinyl alcohol, and then submitting them to pillaring treatment (K. Suzuki et al. Clays and Clay Miner., 1988, 36, 147-152).
Also pillared clays are known which contain simultaneously two intercalating metals, i.e.: gallium and aluminum (A. V. Coelho, G. Ponclet, Appl. Catal., 1991, 77, 303-314), aluminum and iron (F. Bergaya et al., Stud. Surf. Sci. Catal., Preparation of catalysts V., 1991, 329-336), lanthanum and nickel (A.K. Ladavos et al., Stud. Surf. Sci. Catal., Preparation of catalysts V., 1991, 319-328), aluminum and rare earths (J. Serte, Stud. Surf. Sci. Catal., Preparation of catalysts V., 1991, 301-310).
In U.S. Pat. No. 4,666,877, a pillared clay is disclosed which is prepared by treating a smectite with a pillaring agent consisting of a hydroxo-polymer cation with formula EQU [Al.sub.13 O.sub.4 (OH).sub.24 (H.sub.2 O).sub.12 ].sup.7+
in which some aluminum ions have been replaced by transition metals, to yield a polyoxo metallic ion having formula EQU N(Al.sub.12-x M.sub.x)O.sub.4 (OH).sub.24.sup.+a
in which N is selected from Al, Si, Ga, Ge, As, P, Cr, Fe, V, Ru, or Ni, and M is selected from V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Te, Ru, Rh, Pd, Ta, W, Re, Os, Ir, Pt. These materials are useful as catalysts for conventional processes of petroleum transformation, such as cracking, hydrocracking, isomerization, reforming and polymerization.